Progress in drilling techniques in recent years has been remarkable, and the developments of drilling fluid materials suitable for subterranean formulations and techniques for controlling drilling fluids have permitted a high specific gravity under high temperature conditions to be maintained and swelling of Argillaceous Rock layers due to hydration to be controlled. Thus, it becomes possible to drill rock formations.
After completion of an oil well, oil production can be carried out. However, if drilling mud and solution filtering from a cement slurry penetrate into reservoirs which contain clay, the clay swells and plugs pores resulting in a reduction of permeability. Alternatively, when reservoirs of sandstone have not been fully concreted, sand flows into the well with the oil and gas and stands in the casing causing production difficulties such as a reduction in production capacity, etc. As countermeasures to eliminate these production problems, various workovers are carried out. In these cases, in order to minimize formation damage, it has been recently recomended to use completion and workover fluids which do not contain solids such as bentonite or barite, etc., such as drilling mud. Namely, completion and workover fluids used hitherto are those fluids wherein salts for obtaining a specific gravity sufficient to control the ground pressure, hydroxyethyl cellulose as viscosifier, which is not substantially affected by salts, and if necessary, various chemicals are dissolved or suspended in water.
However, the temperature in the well tends to rise as the well becomes deeper, and a reduction in rheological properties of the completion and workover fluids used hitherto results in the problem that the completion and workover fluids do not fully display their capability of cleaning the deposits in the well up to the ground surface. Thus, it has been found that the necessary working time is increased and the deposits can not be completely cleaned out. Accordingly, only when a method that is able to prevent a reduction of rheological properties of completion and workover fluids used hitherto can be found, does it become possible to fully enjoy the beneficial properties of such fluids.